Just a quick note on getting the guest additions to work on Fedora 20 Alpha. In my setup I was getting the below error when installing VBOXADDITIONS_4.2.18_88780.
...Building the VirtualBox Guest Additions kernel modulesThe headers for the current running kernel were not found. If the followingmodule compilation fails then this could be the reason.The missing package can be probably installed withyum install kernel-devel-3.11.3-301.fc20.i686+PAE
Building the main Guest Additions module [FAILED]...As you can see the suggestion above was wrong. After installing the correct package I could install ok.
# yum install kernel-PAE-devel.i686